This invention relates to an automatic noise gate for a synchronizing signal amplifier.
Impulse noise has long been a problem in television receivers, being particularly upsetting to sweep synchronizing and AGC systems. The noise pulses are generally of much greater amplitude than the synchronizing components of the composite video signal. As a result, an AGC system which is allowed to sample these large amplitude noise pulses will develop an output control voltage which tends to rapidly decrease the gain of the IF amplifier. The result of this AGC control voltage change is an undesirably attenuated composite video signal.
One effect of this rapid attenuation of composite video signal is an intermittent loss of synchronization. When a greatly attenuated composite video signal is applied to a conventional sync separator, the sync separator is unable to immediately respond to the attenuated video signal. The resultant loss of synchronization pulses at the output of the sync separator causes the vertical and horizontal deflection systems to become unlocked.
A well-known method for minimizing the effects of impulse noise on AGC systems is to turn the AGC system off for the duration of the noise pulse. This prevents the AGC system from sampling the large amplitude noise pulses, and, consequently from developing an AGC output voltage which reduces the gain of the IF amplifier.
Turning the AGC system off for the duration of the noise pulse while correcting AGC system problems allows charging of capacitors coupling the composite video signal to sync amplifier and sync separator circuits during the noise pulses to the extent that several sync pulses are dropped following the noise pulse.